In what way is the instinctive connected with the compulsion to repetition? At this point the idea is forced upon us that we have stumbled on the trace of a general and hitherto not clearly recognised—or at least not expressly emphasised—characteristic of instinct, perhaps of all organic life. According to this, _an instinct would be a tendency innate in living organic matter impelling it towards the reinstatement of an earlier condition_, one which it had to abandon under the influence of external disturbing forces—a kind of organic elasticity, or, to put it another way, the manifestation of inertia in organic life.[16] This conception of instinct strikes us as strange, since we are accustomed to see in instinct the factor urging towards change and development, and now we find ourselves required to recognise in it the very opposite, viz. the expression of the conservative nature of living beings. On the other hand, we soon think of those examples in animal life which appear to confirm the idea of instinct having been historically conditioned. When certain fish undertake arduous journeys at spawning-time, in order to deposit the spawn in certain definite waters far removed from their usual habitats, according to the interpretation of many biologists they are only seeking the earlier homes of their kind, which in course of time they have exchanged for others. The same is said to be true of the migratory flights of birds of passage, but the search for further examples becomes superfluous when we remember that in the phenomena of heredity and in the facts of embryology we have the most imposing proofs of the organic compulsion to repetition. We see that the germ cell of a living animal is obliged to repeat in its development—although in a fleeting and curtailed fashion—the structures of all the forms from which the animal is descended, instead of hastening along the shortest path to its own final shape. A mechanical explanation of this except in some trifling particulars is impossible, and the historical explanation cannot be disregarded. In the same way we find extending far upwards in the animal kingdom a power of reproduction whereby a lost organ is replaced by the growth of a new one exactly like it.